Cdl2 layered structure

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Scheme (4) is restricted to Mg, since nd orbitals are called for. Mgl2 -cited as an example in Table 6.3 - has the two-dimensional Cdl2 layer structure described in Section 3.3. 

Many binary Sn(IV) compounds are known. Sn02 has the rutile structure, and SnX2 with X = S, Se, Te the Cdl2 layer structure. SnF4 has a layer structure constructed from corner-sharing octahedra, but other tetrahalides form tetrahedral molecules. The halides are good Lewis acids, especially SnF4, which forms... 

The sulfides have been studied much less than the oxides, but it is clear that many similarities exist. Like the oxides, a number of stable phases exist and nonstoichiometry is prevalent. The most important sulfide is TiS2, which has the Cdl2 layer structure, in which the Ti atoms occupy octahedral sites between alternate layers of close-packed sulfur atoms. The adjacent sulfur layers are able to intercalate Lewis bases such as aliphatic amines. Similar intercalation compounds can be made with MS2 and MSe2 compounds (M = Ti, Zr, Hf, V, Nb, Ta). Many of these compounds exhibit useful electrical properties including superconductivity see Superconductivity). 

A number of zinc hydroxy compounds are based on the Cdl2 layer structure but with zinc ions attached to the outside of the layer. Since the I atoms are close packed in a Cdl2 layer, there are two kinds of site on either side of such a layer for... 

In addition to the rutile and fluorite structures there are some 3D structures in Table 9.15 in which the environments of the ions are less symmetrical. The CaCl2 structure (C) is closer to the ideal h.c.p. AX2 structure (p. 141) than is the rutile structure there is slightly pyramidal coordination of Cl" (Br ) suggesting a tendency towards the Cdl2 layer structure with its much more pronounced pyramidal coordination of the anions. 

Carbon disulfide CS2 has similar bonding to C02, but SiS2 differs from silica in having a chain structure based on edge-sharing tetrahedra, and GeS2 adopts the Cdl2 layer structure with octahedral Ge (see Topic D3). 

Zn and Cd halides (apart from fluorides) are based on close-packed lattices of halide ions, with Zn occupying tetrahedral holes and Cd octahedral ones. The Zn compounds are best regarded as polymeric, whereas those of Cd are prototypes of the important CdCl2 and Cdl2 layer structures. 

The CdCl2 lattice is related to the Cdl2 layer-structure but with the Cl ions in a cubic close-packed arrangement. Examples of compounds adopting this structure are FeCl2 and C0CI2. Other layer structures include talc and mica (see Section 13.9). 

Na, metal Cdl2, layered structure octahedral site, 6-... 

Na, metal Cdl2, layered structure octahedral site, 6-coordinate Ga-doped Si, extrinsic semiconductor Na2S, antifluorite structure perovskite, double oxide CaF2, fluorite structure GaAs, intrinsic semiconductor wurtzite and zinc blende, polymorphs Sn02, cassiterite. 

In crystalline PbCU and PbBr2 each lead atom is coordinated by nine halide ions, six of which lie at the apices of a trigonal prism, with the remaining three beyond the centres of the three prism faces. Pbl2 has the Cdl2 layer structure s. ... 

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